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A target-oriented magnetotelluric inversion approach for characterizing the low enthalpy Groß Schönebeck geothermal reservoir

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SUMMARY

Electrical conductivity is a key parameter for the exploration and characterization of geothermal reservoirs as hot mineralized formation water of active geothermal areas usually exhibits significantly higher conductivity than the surrounding host rock. Here we present results of a magnetotelluric (MT) exploration experiment carried out in the vicinity of the Groß Schönebeck geothermal test site in Northern Germany, where a doublet system of two 4.3-km deep boreholes was drilled to establish an in situ laboratory to investigate the potential for geothermal energy production. Classical 2-D smooth inversion of the MT data, recorded along two profiles, reveals a shallow conductive structure in good agreement with information from regional geology and seismic images. However, at the northernmost part of the profiles, the conductivity models reveal deep-reaching conductive structures, which appear uncorrelated with existing (geophysical or geological) data. Incorporating information from seismics as independent constraints for MT inversions allows us to examine the model space rigorously but target oriented. Employing so-called tear-zone inversions we can effectively derive an alternative class of models, which are consistent with the MT observations but also with the other data sets. We speculate that the zones of high conductivity imaged at reservoir depth are related areas of reduced thickness of the overlaying evapourite layer. The enhanced conductivity can be explained by a higher fracture density in anhydritic layers and/or generally lower resistivity of the pore fluid.

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